The present invention relates to an uninterruptible power supply which connects a load to utility power supply through an AC switch.
AC switches used in conventional uninterruptible power supplies, as disclosed in Japanese Patent Laid-Open No. 5-292686, employ switching element with a self turn-off function or thyristors that are turned off by a forced turn-off circuit.
Another type of AC switch is also used which does not use a turn-off circuit and is naturally turned off as a current flowing in the thyristor falls below a holding current. The use of the AC switch incorporating only such thyristors can reduce the cost of the uninterruptible power supply. However, when this kind of AC switch is used, a period occurs without fail during which two AC powers are parallelly connected to and fed to a load, delaying the turn-off of the AC switch, as indicated in Japanese Patent Laid-Open Nos. 5-30683, 5-176461 and 5-292686.
As shown in FIG. 8A, a conventional thyristor-based AC switch 1 for each phase has antiparallelly connected thyristors SCR1 and SCR2 disposed between a utility or commercial power supply 3 and a power converter 5 for each phase. The power converter 5 has a function of converting an AC power into a DC power and vice versa, and is connected to a DC power supply 7 on a DC side thereof. The electric power from the utility power supply 3 is fed through the AC switch 1 to a load 9, and the output from the power converter 5 is supplied directly to the load 9.
To initiate a switchover between the two power supplies 3, 7, from the power converter 5 side (second feeding path R2) to the utility power supply 3 side (first feeding path R1), an ON signal is applied to each of the thyristors SCR1 and SCR2 making up the AC switch 1 at a predetermined cycle and an OFF signal is given to the power converter 5. At this time the current flowing through the power converter 5 is immediately interrupted and the AC switch immediately turns on, causing the feeding path to be changed over instantaneously.
To switch the feeding path from the utility power supply side (first feeding path R1) to the power converter side (second feeding path R2), an ON signal is applied to the power converter 5 and an OFF signal is supplied to the AC switch 1. However, since the AC switch 1 uses thyristors without the self turn-off function, the current flowing through the AC switch 1 does not fall immediately to zero and thus there is a period when the utility power supply 3 and the power converter 5 are connected parallelly to the load 9. During this period, depending on the voltage relation between the utility power supply 3 and the power converter 5, an uncontrollable cross current flows between the power converter 5 and the utility power supply 3, rendering the voltage applied to the load 9 abnormal. The inexpensive AC switch 1 comprising only the thyristors SCR1 and SCR2 that do not use a forced turn-off circuit, therefore, has a problem that it cannot make an instantaneous switchover of the feeding path.
An object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors as quickly as possible.
Another object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors as quickly as possible by using a power converter.
Still another object of the present invention is to provide an uninterruptible power supply capable of turning off an AC switch made up of thyristors without providing a separate, forced turn-off circuit.
Yet another object of the present invention is to provide an AC switch turn-off method for an uninterruptible power supply capable of turning off an AC switch made up of thyristors without providing a separate, forced turn-off circuit.
The present invention improves an uninterruptible power supply which comprises: a first feeding path to feed an AC power to a load from a utility power supply through an AC switch made up of a pair of antiparallelly connected thyristors; a second feeding path including a voltage control type power converter, the power converter using a DC power supply as its source and performing an inverter operation of converting a DC power into an AC power and feeding the converted AC power to the load; a reference sinusoidal voltage generation circuit to produce a reference sinusoidal voltage; a control command generation means to, based on the reference sinusoidal voltage, output to the power converter a voltage control command to cause the power converter to perform the inverter operation, wherein, while the voltage control command is being output from the control command generation means, the AC switch is turned off and the feeding of the AC power to the load is switched over from the first feeding path to the second feeding path. The AC switch is arranged for each phase. Thus, for a three-phase AC power, three single-phase AC switches are provided, one for each phase.
In the uninterruptible power supply of the present invention, the reference sinusoidal voltage generation circuit includes: a sinusoidal voltage generation means to generate a synchronous sinusoidal voltage Vref1, a high synchronous sinusoidal voltage Vref2 and a low synchronous sinusoidal voltage Vref3, wherein the synchronous sinusoidal voltage Vref1 is synchronized with the utility power supply and, when used as the reference sinusoidal voltage, causes an output voltage of the power converter to become almost equal to a voltage of the utility power supply, the high synchronous sinusoidal voltage Vref2 is in phase with the synchronous sinusoidal voltage Vref1 and higher in amplitude than the synchronous sinusoidal voltage Vref1, and the low synchronous sinusoidal voltage Vref3 is in phase with the synchronous sinusoidal voltage Vref1 and lower in amplitude than the synchronous sinusoidal voltage Vref1; a current detection means to detect a current flowing through the AC switch; and a sinusoidal voltage selection means to select the synchronous sinusoidal voltage Vref1, the high synchronous sinusoidal voltage Vref2 or the low synchronous sinusoidal voltage Vref3 and output the selected voltage as the reference sinusoidal voltage to the control command generation means, wherein when the voltage control command is not output, the sinusoidal voltage selection means selects the synchronous sinusoidal voltage Vref1 and wherein when the voltage control command is output, the sinusoidal voltage selection means selects the high synchronous sinusoidal voltage Vref2 if an output of the current detection means has a positive polarity, selects the low synchronous sinusoidal voltage Vref3 if the output of the current detection means has a negative polarity, and selects the synchronous sinusoidal voltage Vref1 if the output of the current detection means is zero.
In the uninterruptible power supply of the present invention, the reference sinusoidal voltage generation circuit includes: a sinusoidal voltage generation means to generate a synchronous sinusoidal voltage Vref1, a high synchronous sinusoidal voltage Vref2 and a low synchronous sinusoidal voltage Vref3, wherein the synchronous sinusoidal voltage Vref1 is synchronized with the utility power supply and, when used as the reference sinusoidal voltage, causes an output voltage of the power converter to become almost equal to a voltage of the utility power supply, the high synchronous sinusoidal voltage Vref2 is in phase with the synchronous sinusoidal voltage Vref1 and higher in amplitude than the synchronous sinusoidal voltage Vref1, and the low synchronous sinusoidal voltage Vref3 is in phase with the synchronous sinusoidal voltage Vref1 and lower in amplitude than the synchronous sinusoidal voltage Vref1; a current detection means to detect a current flowing through the AC switch; and a sinusoidal voltage selection means to select the synchronous sinusoidal voltage Vref1, the high synchronous sinusoidal voltage Vref2 or the low synchronous sinusoidal voltage Vref3 and output the selected voltage as the reference sinusoidal voltage to the control command generation means, wherein when the power failure detection means does not detect a power failure and the voltage control command is not output, the sinusoidal voltage selection means selects the synchronous sinusoidal voltage Vref1 and wherein when the power failure detection means detects a power failure and the voltage control command is output, the sinusoidal voltage selection means selects the high synchronous sinusoidal voltage Vref2 if an output of the current detection means has a positive polarity, selects the low synchronous sinusoidal voltage Vref3 if the output of the current detection means has a negative polarity, and selects the synchronous sinusoidal voltage Vref1 if the output of the current detection means is zero.
According to the present invention, when the first feeding path is switched over to the second switching path, the reference sinusoidal voltage generation circuit generates a reference sinusoidal voltage that causes the output voltage of the power converter in the second feeding path to apply a reverse bias to the conducting one of the thyristors making up the AC switch in the first feeding path. Hence, the AC switch can be turned off quickly without providing a forced turn-off circuit for the thyristors. Particularly, with this invention, when the AC switch is turned off, no period exists during which the utility power supply and the power converter are connected in parallel, thus ensuring that no cross current flows between the utility power supply and the power converter.
The present invention can also be applied to a normally utility power type uninterruptible power supply which normally supplies electric power to the load through the first feeding path and, in the event a failure of the utility power supply, switches the power feeding from the first feeding path to the second feeding path. In that case, the control command generation means outputs a voltage control command while the utility power failure persists.
The present invention can also be applied to a normally utility power feeding and active filter type uninterruptible power supply which normally supplies power to the load through the first feeding path and operates the power converter as an active filter and, in the event a failure of the utility power supply, switches the power feeding from the first feeding path to the second feeding path. In that case, the control command generation means is made to output a voltage control command to the power converter while the power failure persists in the utility power supply and, when there is no power failure in the utility power supply, output to the power converter a current control command that causes it to operate as an active filter.
The present invention can also be applied to a normally inverter type uninterruptible power supply which normally supplies electric power to the load through the second feeding path and, when the power converter is abnormal, switches the feeding from the second feeding path to the first feeding path. In that case, the control command generation means is made to output a voltage control command at all times when the power converter is normal. When the power converter is abnormal, the AC switch is turned on. In this type of uninterruptible power supply, when the power converter is restored or repaired from the abnormal condition and the first feeding path is switched over to the second feeding path, the control command generation means generates a voltage control command.
The present invention also improves an uninterruptible power supply which comprises: a power failure detection means to detect a power failure in a utility power supply; an AC switch having a pair of antiparallelly connected thyristors, the AC switch being adapted to conduct to feed an AC power from the utility power supply to a load when the utility power supply is normal and, when the utility power supply fails, turn off; a DC power supply; a voltage control type power converter arranged between the AC switch and the DC power supply, wherein when the utility power supply is normal, the power converter performs a rectifier operation of converting the AC power from the utility power supply into a DC power and charging the DC power supply with the DC power and an active filter operation of compensating for a harmonic current and a reactive current flowing into the load and wherein at other times the power converter uses the DC power supply as its source and performs mainly an inverter operation of converting a DC power into an AC power and feeding the converted AC power to the load; a reference sinusoidal voltage generation circuit to produce a reference sinusoidal voltage; and a control command generation means to, based on the reference sinusoidal voltage, output to the power converter a voltage control command to cause the power converter to perform the inverter operation and a current control command to cause the power converter to perform the rectifier operation and the active filter operation.
The sinusoidal voltage generation means may have, but is not limited to, a configuration in which it generates the synchronous sinusoidal voltage Vref1, a high synchronous sinusoidal voltage Vref2 in phase with and a voltage difference command value xcex94V higher than the synchronous sinusoidal voltage Vref1, and a low synchronous sinusoidal voltage Vref3 in phase with and a voltage difference command value xcex94V lower than the synchronous sinusoidal voltage Vref1. This arrangement can easily produce sinusoidal voltages required for various controls. In particular, this arrangement can reliably determine the necessary reverse bias voltage by the voltage difference command value xcex94V, so it is possible to output from the power converter a minimum voltage required to turn off the thyristor at whatever timing the AC switch is to be turned off.
The sinusoidal voltage selection means may also be constructed to have a first selection circuit to select the high synchronous sinusoidal voltage Vref2 when the output of the current detection means has a positive polarity and the low synchronous sinusoidal voltage Vref3 when the output of the current detection means has a negative polarity; a first decision circuit to decide that the output voltage of the first selection circuit is required as the reference sinusoidal voltage when the power failure detection means detects a power failure and the current detection means detects that the current is not zero; and a second selection circuit to select the output voltage of the first selection circuit when the first decision circuit decides that the output voltage of the first selection circuit is required and, at other times, select the synchronous sinusoidal voltage Vref1. In this arrangement, the necessary sinusoidal voltage can reliably be selected with a few selection circuits.
In the invention described above, three kinds of sinusoidal voltages Vref1-Vref3 are the same in phase with each other are used to generate a reference sinusoidal voltage that causes the power converter to produce a reverse bias voltage to be applied to the thyristors making up the AC switch. It is also possible to use three kinds of sinusoidal voltages different in phase from one another to generate a reverse bias voltage. In this invention, the uninterruptible power supply comprises: a power failure detection means to detect a power failure in a utility power supply; an AC switch having a pair of antiparallelly connected thyristors, the AC switch being adapted to conduct to feed an AC power from the utility power supply to a load when the utility power supply is normal and, when the utility power supply fails, turn off; a DC power supply; a voltage control type power converter arranged between the AC switch and the DC power supply, wherein when the utility power supply is normal, the power converter performs a rectifier operation of converting the AC power from the utility power supply into a DC power and charging the DC power supply with the DC power and an active filter operation of compensating for a harmonic current and a reactive current flowing into the load and wherein at other times the power converter uses the DC power supply as its source and performs mainly an inverter operation of converting a DC power into an AC power and feeding the converted AC power to the load; a phase shift type reference sinusoidal voltage generation circuit to produce a reference sinusoidal voltage; and a control command generation means to, based on the reference sinusoidal voltage, output to the power converter a voltage control command to cause the power converter to perform the inverter operation and a current control command to cause the power converter to perform the rectifier operation and the active filter operation.
The AC switch comprises a pair of antiparallelly connected thyristors. This invention uses the phase shift type reference sinusoidal voltage generation circuit to generate three kinds of sinusoidal voltages different in phase from one another. The phase shift type reference sinusoidal voltage generation circuit comprises: a phase shift type sinusoidal voltage generation means to generate a synchronous sinusoidal voltage Vref1, a leading sinusoidal voltage Vref4 and a lagging sinusoidal voltage Vref5, wherein the synchronous sinusoidal voltage Vref1 is synchronized with the utility power supply and, when used as the reference sinusoidal voltage, causes an output voltage of the power converter to become almost equal in amplitude to a voltage of the utility power supply, the leading sinusoidal voltage Vref4 is equal in amplitude to and leading in phase the synchronous sinusoidal voltage Vref1, and the lagging sinusoidal voltage Vref5 is equal in amplitude to and lagging in phase the synchronous sinusoidal voltage Vref1; a current detection means to detect a current flowing through the AC switch; and a phase shift type sinusoidal voltage selection means to select the synchronous sinusoidal voltage Vref1, the leading sinusoidal voltage or the lagging sinusoidal voltage and output the selected voltage as the reference sinusoidal voltage to the control command generation means, wherein when the power failure detection means does not detect a power failure and the voltage control command is not output, the phase shift type sinusoidal voltage selection means selects the synchronous sinusoidal voltage Vref1 and wherein when the power failure detection means detects a power failure and the voltage control command is output, the phase shift type sinusoidal voltage selection means selects the leading sinusoidal voltage or the lagging sinusoidal voltage, whichever is larger than the synchronous sinusoidal voltage if an output of the current detection means has a positive polarity, selects the leading sinusoidal voltage or the lagging sinusoidal voltage, whichever is smaller than the synchronous sinusoidal voltage the if the output of the current detection means has a negative polarity, and selects the synchronous sinusoidal voltage Vref1 if the output of the current detection means is zero.
As described above, even if sinusoidal voltages different in phase are selected to generate a reverse bias voltage from the power converter, the thyristors forming the AC switch can be turned off in a short time. In this invention, there is a period in which the leading sinusoidal voltage Vref4 and the lagging sinusoidal voltage Vref5 are both smaller in absolute value than the synchronous sinusoidal voltage Vref1. But this period is very short and, once this period is passed, either of the leading sinusoidal voltage Vref4 or the lagging sinusoidal voltage Vref5 becomes larger in absolute value than the synchronous sinusoidal voltage Vref1 without fail. Thus, if a power failure should occur in this short period, the AC switch can be turned off with only a small time delay. Therefore no practical problem arises with this invention.
The phase shift type sinusoidal voltage generation means may be configured to produce the synchronous sinusoidal voltage Vref1, the leading sinusoidal voltage Vref4 equal in amplitude to and leading in phase the synchronous sinusoidal voltage Vref1 by a phase difference command value xcex94xcfx86, and the lagging sinusoidal voltage Vref5 equal in amplitude to and lagging in phase the synchronous sinusoidal voltage Vref1 by the phase difference command value xcex94xcfx86. With the phase shift type sinusoidal voltage generation means arranged in this way, a required reverse bias voltage can easily be set by setting the phase difference command value xcex94xcfx86 at a desired value.
The phase shift type sinusoidal voltage selection means selects the leading sinusoidal voltage Vref4, the lagging sinusoidal voltage Vref5 or the synchronous sinusoidal voltage Vref1 and outputs the selected voltage as the reference sinusoidal voltage to the control command generation means, wherein the phase shift type sinusoidal voltage selection means selects the leading sinusoidal voltage Vref4 after the current has changed from the negative polarity to the positive polarity until the synchronous sinusoidal voltage Vref1 reaches a positive peak value, selects the lagging sinusoidal voltage Vref5 after the synchronous sinusoidal voltage has reached the positive peak value until the current changes from the positive polarity to the negative polarity, selects the leading sinusoidal voltage Vref4 after the current has changed from the positive polarity to the negative polarity until the synchronous sinusoidal voltage reaches a negative peak value, selects the lagging sinusoidal voltage Vref5 after the synchronous sinusoidal voltage has reached the negative peak value until the current changes from the negative polarity to the positive polarity, and selects the synchronous sinusoidal voltage Vref1 when the current detection means detects that the current is zero.
The present invention also relates to an AC switch turn-off method for an uninterruptible power supply, wherein the uninterruptible power supply has a first feeding path to feed an AC power to a load from a utility power supply through an AC switch made up of a pair of antiparallelly connected thyristors and a second feeding path including a voltage control type power converter, the power converter using a DC power supply as its source and performing an inverter operation of converting a DC power into an AC power and feeding the converted AC power to the load. In this invention, when the power feeding to the load is switched over from the first feeding path to the second feeding path, a reverse bias voltage from the power converter is applied to a conducting one of the thyristors to forcibly turn off the conducting thyristor. With this, the AC switch can be turned off quickly without providing a forced thyristor turn-off circuit.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.